1. Field of the Invention
The invention relates generally to processes for producing CaF2 crystals. More specifically, the invention relates to a nucleant seed for epitaxial growth of single-crystal CaF2.
2. Background Art
Single-crystal CaF2 is commonly grown using the Bridgman-Stockbarger crystal growth process. For epitaxial growth of CaF2, the process starts, as illustrated in FIG. 1, with a seed crystal 2 made of CaF2 and having the desired crystallographic orientation. For deep-ultraviolet microlithography applications, for example, the desired crystallographic orientation is  less than 111 greater than , i.e., cubic (octahedral or cubic forms) crystal structure. The seed crystal 2 is placed at the base of a crucible 4. A starting material 6 comprising CaF2 powder (or beads) is placed in the crucible 4, on top of the seed crystal 2. The crucible 4 is then placed in a vertical furnace 8 and heated to a temperature sufficient to melt the starting material 6. To prevent oxidation of the starting material 6 and the components of the furnace 8, the furnace 8 is typically maintained under vacuum and/or the process is carried out in an inert atmosphere.
After melting the starting material 6, the crucible 4 is moved downwardly at a predetermined rate (typically 0.3 to 5 mm/h), from a hot zone 10 into a cold zone 12. An insulating barrier 14 separates the hot zone 10 from the cold zone 12. FIG. 2 shows a typical temperature distribution along the vertical axis of the furnace (8 in FIG. 1). A single crystal of CaF2 forms on the seed crystal (2 in FIG. 1) when the molten material reaches the zone 12 in which the furnace temperature is below the melting point of CaF2. The CaF2 crystal front propagates inside the crucible 4, within the material 6, as long as the crucible 4 is caused to move downwardly. The CaF2 crystal conforms to the crystallographic orientation of the seed crystal 2 as it propagates inside the crucible 4.
To enhance the optical properties of the CaF2 crystal, a scavenger is typically added to the starting material 6 to remove oxygen and hydroxyl ions. These impurities have been known to reduce transmission in the deep-ultraviolet region. The most common scavenger used is PbF2. PbF2 is solid and can be added directly to the starting material 6. Typically, a specific amount of PbF2, typically 1 to 2% by weight, is mixed into the starting material 6. The mixture is then gradually heated to approximately 800xc2x0 C. to 900xc2x0 C., at which point PbF2 reacts with the starting material 6 to form PbO. After the reaction is complete, the more volatile PbO is evaporated from the mixture by heating the mixture to the melting point of CaF2 or higher. In an attempt to remove as much of the PbO as possible through volatization, the CaF2 melt may become overheated and cause the seed crystal 2, which is also made of CaF2, to completely melt and lose its crystallographic orientation.
In one aspect, the invention relates to a nucleant seed for epitaxial growth of single-crystal CaF2 which comprises SrF2. In some embodiments, a second fluoride is substituted in the SrF2 structure, the second fluoride being selected from the group consisting of YF3, LaF3, rare-earth fluoride, and combinations thereof. In some embodiments, the rare-earth fluoride comprises one selected from the group consisting of YF3, LaF3, CeF3, NdF3, PrF3, DyF3, SmF3, EuF3, TbF3, and GdF3.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.